The membrane protein Fas/Apo-1/CD95 signals programmed cell death or apoptosis in activated T lymphocytes. Vascular smooth muscle cells (SMCs) bear markers of programmed cell death or apoptosis in advanced atherosclerotic plaques that contain immune cells eg, macrophages and T lymphocytes. This study tested the hypothesis that the Fas death-signaling pathway contributes to apoptosis of SMCs exposed to proinflammatory cytokines produced by these immune cells during atherogenesis. All atherosclerotic plaques examined (n=14) contained immunoreactive Fas. The majority of the Fas⁺ SMCs localized in the intima of the plaques, whereas the medial SMCs expressed Fas antigen less prominently. Double staining for DNA fragments (TUNEL) and Fas or cell identification markers colocalized Fas with TUNEL⁺ SMCs in the areas that contained CD3⁺ T cells and CD68⁺ macrophages, suggesting a role for Fas in the induction of SMC apoptosis by activated T cells during atherogenesis. In culture, stimulation with interferon-γ, tumor necrosis factor-α, and interleukin-1β increased expression of Fas in SMCs. Incubation with an activating anti-Fas antibody triggered apoptosis of the cytokine-primed but not the untreated SMCs, as demonstrated by TUNEL and electrophoresis of oligonucleosomal DNA fragments. These data suggest that activation of the Fas death-signaling pathway contributes to the induction of SMC apoptosis during atherogenesis and furnish a mechanism whereby immune cells and their cytokines promote this cell death process related to vascular remodeling and plaque rupture.